1. Field of the Invention
2. Description of the Prior Art
Such a pump can serve in particular as a prefeed pump for a high-pressure fuel pump, and the fuel is furnished to it by the prefeed pump at a pressure of about 6 bar. The high-pressure fuel pump then generates a pressure, which can be on the order of magnitude of as high as 1800 bar, of the kind used in a so-called common rail injection system.
The geared pump is driven at the same rpm as the high-pressure fuel pump and must furnish a sufficient quantity of fuel already when the engine is at its starting rpm. For this reason, it is necessary that the gear wheels run with as little play relative to the housing as possible and that the wrap length of the two gear wheels, that is, the angular range, over which the interstices between teeth, which are filled with a fuel to be pumped, between the intake side and the compression side of the geared pump are sealed off by the housing, must also be as great as possible. At maximum engine rpm, however, the geared pump must not pump an excessive fuel quantity. Instead of a complicated valve control for quantity regulation, typically a throttle is used on the intake side and defines this feed quantity. As a consequence, when a certain feed quantity is reached, the interstices between teeth are no longer completely filled with fuel.
If such an interstice between teeth, which is not completely filled with fuel, on the compression side of the pump emerges from the housing into the pressure chamber, there is the danger of cavitation damage at the tooth flanks of the gear wheel teeth or at the housing. For this reason the groove is provided, which is intended to enable the most continuous possible pressure increase in the interstice between teeth that is not completely filled with fuel. The groove functions like a throttle, which enables a controlled return flow of fuel from the compression side of the pump into the interstice between teeth located in the vicinity of the groove.
A disadvantage of the fuel pumps known until now is that a groove extending over a comparatively large angular range was necessary if cavitation damage even at high rpm is to be prevented. The great angular length of the groove, however, means that the wrap angle between the housing and the gear wheel decreases, resulting in a reduced feed quantity at lower rpm.
The object of the invention is to refine a geared pump of the type defined at the outset such that even at low rpm a large feed quality is attained, while at the same time at high rpm, cavitation damage is avoided.
In the geared pump of the invention the groove forms a kind of antechamber, which communicates with the compression side through the comparatively narrow gap that is formed in a first portion between the bottom of the groove and the tips of the gear wheel teeth. At high rpm, the narrow gap in conjunction with the overflow cross section, which is formed in the region of the second portion of the groove, leads to a continuous pressure increase in whichever interstice between teeth is just now opening toward the groove. The groove has a total length over a comparatively small angular range, resulting in a large wrap angle between the gear wheel and the housing, which is advantageous for the sake of the feed quantity at low rpm.